Comparators have been used particularly in position measuring devices to detect when the outputs of a scanning unit, i.e. scanning signals, are in a relatively high state so that digital outputs can be correctly represented. Typically the inputs to the comparators have been generated in one of three ways. One way is to provide as an input to the comparators a fixed voltage as the reference signal and provide the scanning signals to the other input of the comparators. The reference voltage has a value that is selected to be at the midpoint of the photodetector voltage swing. By setting the reference voltage at this midpoint the comparators can detect whether the scanning signals are at a relatively high state or low state compared to the reference voltage. For example, when the scanning signal is connected to the positive input of the comparator and the reference voltage is connected to the negative input, the output signal will be high when the scanning signal is higher than the reference voltage. When the input signal is comparatively lower than the reference voltage, the output signal will be low.
Typically a scanning unit will have a plurality of scanning elements, for example three, that produce a plurality of scanning signals. A disadvantage of setting the reference voltage at a midpoint is that it may be difficult to find a reference voltage that works for all scanning elements due to uneven illumination by a light source. This can result in excessive labor at the time of manufacture as the proper value must be determined, and the appropriate circuit adjustments made. In addition, this reference voltage will be fixed, and so will not accommodate scanning signal degradation, which can occur as the light source ages or when the operating environmental temperature varies. As illumination degrades, the scanning signals which are output from the photodetectors will diminish, and there may come a time when one or more no longer exceeds the reference voltage. At this point the comparator circuit ceases to function properly.
An improved method is to replace the fixed reference voltage with a reference voltage generated by an additional optical element. This element is always illuminated, and so it will provide a constant voltage which can be used as a reference voltage to the comparator. Although this approach allows for variations in illumination due to environmental temperature change or the age of the light source, it is more expensive and consumes more surface area because of the need for an additional element and requires a larger area of illumination from the light source. In addition, cross-talk from adjacent tracks on the code disk can produce fluctuations in the reference signal which can cause false triggering of the comparator.
A third method uses two optical sensors, one of which will always be off when the other is on. By using these as inputs to a comparator, a clean switching signal can be developed at the output. This signal is very robust due to the large signal difference developed by the complimentary optical signals. This approach allows for light degradation, and is relatively insensitive to modest levels of crosstalk. A disadvantage is that now two sensors are required to generate each output signal which increases manufacturing costs. This approach also requires an even larger illuminated area and associated space on the code disk. This can be a severe disadvantage as it is desirable to make encoder dimensions smaller.
It is thus desirable to provide a position measuring device that provides a reference signal which is compared to the scanning signals that allow for light degradation, that is relatively insensitive to cross talk and does not require an additional sensor or an enlarged illuminated area and associated space on the code disk.